how to pass columns instead of passing data in oracle nested tables

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SQL

Oracle

SET SERVEROUTPUT ON
DECLARE
TYPE LOCATION_TYPE IS TABLE OF VARCHAR2(30);
OFFICES LOCATION_TYPE;
TABLE_COUNT NUMBER;
BEGIN
OFFICES := LOCATION_TYPE('BOMBAY','TOKYO','SINGAPORE','OXFORD');
TABLE_COUNT := OFFICES.COUNT();
FOR I IN 1..TABLE_COUNT LOOP
DBMS_OUTPUT.PUT_LINE(OFFICES(I));
END LOOP;
END;

HERE I AM GETTING RESULT PERFECTLY
-------------------------------------------------------------------

SET SERVEROUTPUT ON
DECLARE
TYPE CONTRACT_TYPE IS TABLE OF CONTRACTREPO.CONTRACTS.CONTRACT_ID%TYPE;
MY_COLLECTION CONTRACT_TYPE;
TABLE_COUNT NUMBER;
BEGIN
MY_COLLECTION := CONTRACT_TYPE();
TABLE_COUNT := MY_COLLECTION.COUNT();
FOR I IN 1..TABLE_COUNT LOOP
DBMS_OUTPUT.PUT_LINE(MY_COLLECTION(I));
END LOOP;
END;

BUT HERE I WANT TO PASS A COLUMN NAME AND DISPLAYING RESULT BUT HERE WHEN
I PASSED COLUMN NAME I AM GETTING ERROR HOW CAN I OVER COME THIS